1. Field of the Invention
The present invention relates to an apparatus for a stepwise advancing of a web-shaped workpiece, which apparatus includes two workpiece advancing units operative to clampingly receive a workpiece therebetween, of which two workpiece advancing units the first workpiece advancing unit is periodically movable towards the second workpiece advancing unit such to periodically clamp a workpiece therebetween and being periodically movable away from the second workpiece advancing unit such to release the workpiece, and which apparatus includes further a drive shaft and a first intermediate drive transmitting assembly driven by said drive shaft and drivingly connected to said first workpiece advancing unit, and a second intermediate drive transmitting assembly driven by said same drive shaft and drivingly connected to said second workpiece advancing unit.
2. Description of the Prior Art
Such apparatuses find application for instance as so-called feeding or advancing units in punch presses. They are used to slide a web-shaped workpiece stepwise through a punch press, whereby during the respective rest periods of the workpiece during this stepwise feeding the foreseen working thereof is made, such as stamping, embossing, etc. as known in the art.
The moving sequence of such apparatuses for a stepwise advancing of a web-shaped workpiece can be divided into a number of individual steps as follows. It shall be assumed that the upper workpiece advancing unit and the lower workpiece advancing unit comprise each an advancing roller structured in accordance with known designs, such as e.g. disclosed in the Swiss patent specification 543 932. In operation these two rollers make an oppositely directed oscillating movement. Hereby the lower roller or feeding roller, respectively, is stationary, which means it is rotatably supported in the respective frame of the machine and makes merely oscillating movements. The upper roller, i.e., feeding roller, makes the same oscillating movements as the lower feeding roller, is, however, additionally arranged such that it is movable towards the lower feeding roller and away thereof such to clamp and release, respectively, a respective workpiece. Accordingly, the upper feeding roller is subject to two movements, namely on the one hand to the oscillating movement and on the other hand to the up-and-down movement. The individual steps of the operation proceed as follows, whereby in order to simplify the explanation the action of the pressing bar, which as such is known in such apparatuses, is deleted. An initial state shall be assumed that the web-shaped workpiece is at rest and a stamping operation has just been terminated. The tool, i.e., its arresting or guiding, respectively, pins release the web. Now, the upper feeding roller which is not oscillating at this instance is lowered onto the web which accordingly is clamped between the upper feeding roller and the lower feeding roller which at this instance is also not rotating, at rest. The web is now clamped and now the two feeding rollers begin to rotate and push or feed, respectively, the web along a predetermined distance and come again to rest. The upper roller rises, the tool closes down onto the web and again one punching operation step is made. Simultaneously to this punching the upper roller which has been lifted off and the lower roller rotate in an opposite direction back into their initial position such that after an anewed releasing of the web by the tool, the next following feeding step can proceed.
For reasons of the disclosure, it shall be noted additionally that a variety of designs of such feeding apparatuses is known in the art. According to one design, both feeding rollers are driven. According to other designs only the lower feeding roller is driven and the upper feeding roller is supported for free rotation such that it is rotated only during the feeding step due to the counterforce generated during the clamping of the workpiece. A further design includes the so-called pliers or clamp feed. On this design an upper clamping part forming the jaw of the "pliers" is pressed for instance also by means of a roller against a lower, slide or carriage shaped part such that the web-shaped workpiece is clamped between these two parts, whereby the oscillating movement for the feeding of the workpiece and for the returning movement of the clamping pieces into their initial position proceeds rectilinearly.
The rollers or clamps, respectively, of these known feeding apparatuses are as a rule driven by means of crank drive assemblies. Due to this design of the drive the time of rest at the respective end position of the oscillating movement, i.e., during the time span of the reversing of the direction of the rotational movement or rectilinear movement, respectively, is extremely short and taken relative to the crank drive it amounts to only very few degrees of the angle. During this phase of movement or work, respectively, the acceleration and accordingly deceleration of the structural members being moved attain maximal values. Conclusively, the so-called dynamic deformation due to the elasticity of the respective structural units attains also the maximal value. Due to economical reasons, the rotational speed of punch presses is steadily increasing and accordingly the rotational speeds of the feeding apparatuses are also steadily increasing and accordingly the deformation of the parts of the apparatus as well as of the web-shaped workpiece is the longer the more subject to an increase. The seeming resting period is in high-speed punch presses so short that a complete attenuation or disappearance, respectively, of the elastic deformation of parts of the apparatus as well as of parts of the workpiece, although they may be extremely small, as well as a complete attenuation or disappearance, respectively, of possible oscillations of machine parts and of the workpiece can no longer occur completely. The result thereof is an unwanted change of the respective feeding length and also unprecise products.
Similar difficulties are encountered at the controlling of the movement of the clamping members or upper feeding rollers, respectively, which, as is well known, can only be lowered down onto the workpiece and raised off, respectively, therefrom during the time span when no feeding movement occurs. Due to the mentioned short resting phase, the respective clamping members (upper roller, clamp) must be lowered at an extremely high-speed down onto the workpiece. This leads, however, to a high striking or impinging speed, respectively, of a respective arresting member onto the workpiece, i.e. a high striking energy is present due to which specifically web-shaped workpieces consisting of a relatively soft material may be deformed plastically and accordingly permanently.